1. Field of the Invention
The present invention relates to a process for forming a solder mask for the fabrication of printed circuit boards or wiring boards or the like, and an apparatus for carrying out the process for forming the solder mask. The present invention also relates to a process for forming an internal dielectric layer provided with an electric-circuit pattern.
2. Description of the Background Art
In the printed circuit or wiring board industry, for example, a solder mask ink is increasingly used for the fabrication of printed circuit boards (PCBs) or wiring boards, driven by the increased demand for the miniaturization of various devices, which necessitates the use of high-density circuitry in printed circuit boards or printed wiring boards.
Furthermore, in accordance with the demand for the miniaturization of various devices, many of parts to be mounted thereon are of a ball grid array (BGA) type, and accordingly more and more strict requirements are being demanded for PCBs.
Under such circumstances, for instance, a liquid alkaline development type solder mask ink is currently in general use for the formation of solder masks for PCBs, and a so-called “dry film” is also used therefor. In the formation of solder masks, for example, the following requirements are demanded for the solder mask of PCBs:                1. Filling of blind via-holes and through-holes and tenting thereof be complete.        2. The flatness of the solder mask after the formation thereof be as high as possible, for instance, ±5 μm or less.        3. Problems such as a so-called “white haze” problem be not caused when a non-contact type exposure unit and a photographic dry plate are used, which are for attaining high resolution and high precision exposure.        
Complete filling of blind via-holes and through-holes and 100% tenting are difficult to attain by coating on the PCBs the liquid alkaline development type solder mask ink, even though such a liquid solder mask ink is currently coated by various coating methods such as screen printing, curtain coating, spray coating, and roll coating. In contrast to this, complete filling and tenting can be almost carried out by laminating a dry film on PCBs by use of a vacuum laminator.
As to the flatness of the solder mask after the formation thereof, it is difficult to attain the required flatness by use of the solder mask ink due to the presence of copper patterns and via-holes in the PCBs. In contrast to this, the dry film can attain the required flatness.
The currently available alkaline development type solder mask ink utilizes radical polymerization and therefore unless it is protected from oxygen, oxygen hindrance to the formation of solder mask takes place. More specifically, when the alkaline development type solder mask ink is exposed to oxygen, the surface of the solder mask formed become white in color and the function of the solder mask is impaired. This problem is referred to as “white haze” phenomenon.
In order to avoid such oxygen hindrance, a negative film is used since it can be brought into close contact with the coated solder mask ink, and therefore the coated solder mask ink can be protected from oxygen. However, for the purpose of obtaining high resolution and high precision exposure, the negative film is not always suitable since the negative film itself elongates or shrinks while in use and causes a problem with positional accuracy and precision in the formation of a solder mask.
In order to attain high resolution and high precision exposure by avoiding the problem with positional accuracy and precision of exposure, a non-contact exposure unit such as a step-and-repeat type non-contact exposure unit and a photographic dry plate are considered to be preferable for use. However, when the non-contact exposure unit and the photographic dry plate are used, a certain space is inevitably formed between the dry plate and the coated solder mask ink. This space causes the above-mentioned oxygen hindrance.
In this sense, the solder mask ink in current use is not suitable for the formation of a solider mask with high resolution and high precision, particularly when the non-contact exposure unit and the photographic dry plate are used. In contrast to this, the dry film has almost no problem with respect to the formation of a solder mask with high resolution and high precision, since the dry film is protected from oxygen while in use for the fabrication of the solder mask.
When the solder mask ink is used, a solder mask is generally prepared, for example, by the steps of coating the solder mask ink onto a PCB, drying the coated solder mask ink to form a photoimageable resist layer on the PCB, exposing the photoimageable resist layer imagewise so as to correspond to a solder mask to be formed, developing the exposed photoimageable resist layer to a developed resist layer, and thermally curing the developed resist layer to form a solder mask on the PCB.
Such a solder mask ink can be used for 6 months when preserved in the dark at temperatures below 20° C. When the solder mask ink is used, the work size and the thickness of the solder mask can be changed as desired. However, as mentioned above, when such a solder mask ink is used, complete filling of through-holes and blind via-holes with the ink is difficult to perform, and complete tenting of through-holes is also difficult to perform. Furthermore, the formation of voids is inevitable.
The flatness obtained by the use of the solder mask ink is in the range of ±20 μm or more. Furthermore, as mentioned above, when a non-contact exposure apparatus and a photographic dry plate are used, the surface of the solder mask tends to be whitened due to the above-mentioned “white haze” phenomenon which is caused by the oxygen hindrance.
A dry film which is currently commercially available has a multi-layered structure as shown in FIG. 11, and such a dry film, when commercially supplied, is in the form of a roll film.
In FIG. 11, reference numeral 10 indicates a carrier film, which may be made of, for example, a polyester film; reference numeral 20, a resist layer which may be, for example, a polymerizable layer serving as a photoimageable solder mask layer; and reference numeral 30, a protective layer for protecting the resist layer 20, which protective layer may be, for example, a polyethylene film.
Generally, such a dry film roll is preserved in the dark in a frozen state, for example, at −20° C., and is thawed out before use.
When such a dry film is used in practice, for example, the following steps are generally taken:
To begin with, the frozen dry film is thawed out as mentioned above. When the temperature thereof has reached, for example, room temperature, the protective layer 30 is peeled away from the resist layer 20.
A portion with a desired size is then cut off the dry film from which the protective layer 30 has been peeled away.
The cut portion is then superimposed on a PCB in such a manner that the upper surface of the resist layer 20 comes into contact with the PCB, and one end portion 20c of the resist layer 20 is tacked to the PCB by a tack bar B as shown in FIG. 13.
The resist layer 20 is then exposed to light imagewise corresponding to a solder mask to be formed through the carrier film 10. The carrier film 10 is then peeled away from the resist layer 20. The exposed resist layer 20 is then preheated, cooled to room temperature, and developed by spraying thereon an aqueous solution of sodium carbonate, followed by washing the resist layer 20 with water by spraying water thereon. The thus developed resist layer 20 is then thermally cured, whereby a solder mask is formed on the PCB.
Generally, such a dry film is usable for 3 months when preserved in the dark at temperatures below −20° C., for 2 months when preserved in the dark at temperatures below 0° C., and only for 2 days when preserved in the dark at temperatures below 20° C.
When such a dry film is used, complete filling of through-holes and blind via-holes and complete tenting of through-holes are possible, without the formation of voids.
The flatness obtained by the use of the dry film is in the range of ±5 μm or less. Even when the non-contact exposure apparatus or the photographic dry plate is used, no “white haze” phenomenon takes place.
However, the work size and the thickness of the solder mask cannot be changed easily as desired. In order to change the work size and the thickness of the solder mask, a number of different dry films are needed. Changing the work size and the thickness of the solder mask by use of a number of different dry films is not a practical procedure.